1. Field of the Invention
This invention relates to battery chargers for charging batteries incorporated in portable audio devices such as portable telephones. This invention also relates to audio systems comprising portable audio devices and battery chargers.
2. Description of the Related Art
Until now, no battery chargers are designed and developed for portable audio devices such as portable telephones (e.g., cellular phones) incorporating amplifiers for use in reproduction of hi-fi (or high-fidelity) audio signals. Portable audio devices can be used as constituent elements of audio systems. FIG. 9 shows a typical example of an audio system incorporating a portable audio device. Specifically, the audio system of FIG. 9 is constituted by connecting together a portable audio device 1 with speakers 2A and 2B incorporating amplifiers (not shown). These speakers 2A and 2B are connected together by a speaker cord 3, while the portable audio device 1 is connected with the speaker 2A by an analog audio signal cord 4. In addition, the speaker 2B has a tone volume control for adjusting the tone volume thereof. Further, both the portable audio device 1 and the speaker 2A are connected with AC/DC convertible power adapters 6 via power supply cords 5.
Japanese Unexamined Patent Publication No. Hei 10-313257 discloses a battery charger having a radio reception function for a portable telephone. This battery charger has a loudspeaker to provide a loudspeaker function of radio sound. Japanese Unexamined Patent Publication No. 2000-286929 discloses a battery charger for a portable telephone that allows conversation while the charging operation is in progress.
In order to constitute the audio system incorporating the portable audio device as shown in FIG. 9, both the portable audio device 1 and the speaker 2A incorporating an amplifier require power sources independently. Therefore, the audio system may have a temporal appearance using cords and is not compact. That is, this audio system lacks comprehensive operability.
In addition, it may be difficult to use the aforementioned audio system in situations in which the user is listening to music from speakers while the charging operation of a battery of a portable audio device is in progress using an indoor AC power source.
The battery charger disclosed in Japanese Unexamined Patent Publication No. Hei 10-313257 is designed under the precondition where analog audio signals given from a portable telephone incorporating a radio are amplified by a linear amplifier incorporated in a battery charger. Herein, the power amplification efficiency of the linear amplifier may normally range from 20% to 25%, so that it may require power consumption ranging from 1.7 W to 2.3 W in order to produce outputs of (0.5 W to 1.0 W)xc3x972 channels, whereas the remaining power ranging from 85% to 70% may cause heat in the amplifier. For this reason, a heat radiation board or space should be necessary in the battery charger in order to ensure sufficient heat radiation or dissipation in the amplifier. This causes problems in downsizing of the battery charger and in increasing of the amplifier output.
In addition, the aforementioned battery charger is designed to input analog audio signals. Therefore, when the source voltage of the portable telephone is reduced for the purpose of the conservation of electric energy, amplitudes of analog audio signals should be correspondingly reduced so that the portable telephone could not provide high sound quality.
Because of the relatively large power consumption of the amplifier, in order to charge the battery in a short period of time by securing sufficient charging power, it is necessary to employ the following measures:
(i) To increase the size of the power source circuit and the size of the battery charger; and
(ii) To use a small-output amplifier in order to reduce electric power consumed in the amplifier.
The battery charger disclosed by Japanese Unexamined Patent Publication No. 2000-286929 incorporates a speaker and a small-output linear amplifier for the purpose of conversation using the portable telephone. Therefore, this battery charger is not appropriate for the realization of a high sound quality based on digital audio signals because it provides small amplifier output and operates based on analog signals only.
It is an object of the invention to provide an audio system incorporating a battery charger that can charge a battery of a portable audio device such as a portable telephone using an indoor AC power source while allowing reproduction of audio signals by speakers.
A battery charger is used for charging a battery incorporated in a portable audio device such as a portable telephone, and it comprises a connector and a circuit block for performing charging operation with the battery of the portable audio device via the connector. The circuit block contains a digital power amplifier that ensures digitization of audio signals transmitted thereto from the portable audio device via the connector. The battery charger can be connected with speakers, which produce sounds based on digital audio signals amplified by the digital power amplifier. Since the digital power amplifier has a good efficiency in power consumption and produces a very small amount of heat during operation, the battery charger can be considerably downsized, and it can reliably ensure a high quality reproduction of audio signals during charging operation for the battery of the portable audio device in progress.
In the above, the battery charger uses a 16-pin connector that includes at least one charging terminal and a plurality of terminals for use in transmission of signals with respect to the portable audio device. In addition, a selector can be installed in the battery charger to allow the user to manually select a desired terminal for transmission of signals. The digital power amplifier may include a PCM/PWM converter, a pulse-width modulator, a power driver, and switching transistors, for example.
An audio system can be constituted using a portable audio device and a battery charger. Herein, the battery charger merely requires a single power source for both the charging operation and playback operation with respect to the portable audio device.